Purge means for storage tank



May 30, 1961 s. BEcKwlTH 2,986,010

PURGE MEANS FOR STORAGE TANK Filed July 7, 195s l I l l l l l UnitedStates Patent() PURGE MEANS FOR STORAGE TANK Sterling Beckwith, LakeForest, Ill., assignor, by mesne assignments, to Couch InternationalMethane Limited, Nassau, Bahamas, a corporation of the Bahamas FiledJuly 7, 195s, ser. No. u747,015

Claims. (ci. 624s) This invention relates to a tank construction for usein the storage of liqueied natural gas and the like hydrocarbons and itrelates more particularly to a storage facility of the type describedwhich operates automatically to purge the container of volatile materialwhen the container is emptied of the liquid.

Referring to the storage of liquetied natural gas or methane in largecontainers at about atmospheric temperature, the vapor continuouslygiven off by the liquid upon absorption of heat from the atmosphere orotherwise represents a gas capable of combustible mixture with air oroxygen. Ordinarily, when liquefied gas is present in the container aslight positive pressure will be maintained in the container to preventthe inliltration of air to form a combustible or explosive mixture.When, however, the liquefied gaseous material is substantiallycompletely removed from the container, as by evaporation or by liquidremoval, the means for maintaining positive pressure within the tankwill no longer be available so that the tank will begin to breathe airinto and out of its interior. Under such circumstances, a mixture of airand gaseous vapors can be formed which Will be capable of combustion.

It is advisable to avoid the build-up of such conditions as an incidenceto the storage and transportation of liquetied natural gas or other lowboiling liqueiied combustible gases and it is an object of thisinvention to provide means within the storage tank which is alwayseective to prevent the development of such undesirable conditions.

It is an object of this invention to purge the tank of combustiblegaseous vapors when the tank is emptied or to effect dilution of thevapors and the admixture of an inert gas to form an atmosphere which isincapable of combustion.

A further object of the invention is to introduce an inert gas into thestorage space when the tank is emptied of liquid to displace the vaporsfrom the container or form a mixture therewith which is incapable ofcombustion and it is a related object to bring such conditions intooperation automatically in response to the removal or loss of liquidcargo from the tanks.

These and other objects and advantages of this invention willhereinafter appear and for purposes of illustraa tion, but not oflimitation, an embodiment of the invention is shown in the accompanyingdrawing in which- Fig. 1 is a sectional elevational plan view of theinterior of a liquid storage tank embodying the features of thisinvention, and

Fig. 2 is an enlarged elevational view partially in section of the purgemeans within the tank.

The invention can best be described with reference to the storage oflarge volumes of liqueed natural gas 10 housed within a container 12arranged Within another container 14 having a spaced relationshiptherebetween which is packed or otherwise provided with insulation 16.The inner tank 12 is provided with an inlet 18 for lling the tank withthe liquid and an outlet 20 having a tube 22 extending to the bottom ofthe tank for the discharge of liquid. It is also provided with one ormore vapor outlets 24 in communication with the interior at the top ofthe tank for the release of vapors that are formed. It will beunderstood that the storage tank may be constructed without the innershell 12 when the insulation layer is capable of blocking penetration ofthe liquid to the outer wall 14 of the structure.

The liquefied natural gas 10 has a boiling point at about the criticaltemperature for methane (-258 F. at atmospheric pressure). It will beunderstood that the concepts of this invention which will hereinafter bedescribed have application also to use with other cold boiling liquefiedgases and particularly to such liquids formedv of combustible gases orgases which are capable of forming dangerous or undesirable mixtureswith air or oxygen.

Such liquids vaporize upon the absorption of heat from the ambientatmosphere and the vapors normally till the available space within thetank above the liquid level. It is undesirable to have such vaporscollect in the tank when the pressure conditions existing within thetank are able to fall to atmospheric pressure or below whereby air canbe drawn into the tank for admixture with the vapors. When, as ispreferred, the tanks are maintained at slight positive pressure whenlilled with the liquid, such undesirable conditions will be capable ofdevelopment only where for one reason or another the liquid is eitherremoved or allowed to escape from the tank.

Thus it is desirable to be able to purge the tank of such vapors whenthe tank is emptied or else to dilute the vapor concentration with aninert gas in an amount which will avoid the development of undesirablemixtures with air. It is desirable also to effect such removal ordilution automatically in response to the development of conditionswhich would otherwise lead to an inflow of air or oxygen. n

In accordance with the practice of this invention, the tank is purged ofthe hydrocarbon or other vapors to eiect removal or dilution by theintroduction of an inert gas, represented by carbon dioxide, which isreleased only in response to the increase in temperature in the tankwhen the tank becomes emptied of the colder boiling liquid. Thedifference in temperature between the temperature of the liquelied gasand the temperature of the vapor after the liquid is removed operates toprovide the control for the release of the purging vapors automaticallyin response to the development of the required conditions.

For this purpose, carbon dioxide 30 in solid form is housed Within asealed container 32 xed, as by strips 34, to the bottom 36 of the liquidcontaining tank 12.` Carbon dioxide is present in the container 32 in anyamount to purge the tank of vapors or otherwise cause dilution to theextent that a safe and inert mixturel is formed when released from thehousing 32 into the interior of the 46 of the tank. The outwardlyextending portion 48 in communication with the opening and thedownpipeisVV provided with a removable cover plate 50 and with apressure gauge 52 connected by line 54 to an opening in the cover 50 andtted with a gate valve 56. The cover and connections are also providedwith sealing members and gaskets for maintaining a desired sealingrelationship f throughout the length of the downpipe in communicationwith the container. The dimension of the cover 50 and L Patented May 30,1961 the downpipe 40 is such as to permit small pieces of solid carbondioxide (Dry Ice) to be fed into the downpipe for charging the container32. A straight line connection is desired to enable use of an elongateram to effect the desired displacement of the solid carbon dioxidethrough the feed pipe.

The container is also provided with one or more outlets 60 each of whichis fitted with a pressure relief valve 62 adapted to remain closed untilthe pressure within the container 32 reaches a set maximum, such as 10p.s.i. The pressure responsive valves 62 have outlets communicating withtubings 64 to distribute the carbon dioxide gas issuing from thecontainer 32 into the interior of the tank 12. In the illustratedmodification, two such outlets and pressure relief valves are shown withthe tnbings 64 extending about 90 degrees apart from the cover.

In operation, solid carbon dioxide in particulate form is fed into thehousing through the inlet 44 at the top and then the cover plate 50 issecured in place to seal the container. The liquefied natural gas in thetank will be at a temperature considerably below the solidificationtemperature for the carbon dioxide, such as at a temperature of about240 to -258 F. for liquefied natural gas at atmospheric pressure, orpreferably slightly above. Thus the carbon dioxide will be retainedwithin the container in solid form and any carbon dioxide vaporsgenerated will be resolidified as long as cold liquid is in contact withthe container to maintain the interior at a temperature below thefreezing point of carbon dioxide (-78 F.). However, when insufficientliquid remains in the tank to maintain the desired cold temperature, thetemperature in the tank will slowly rise until it reaches a temperatureabove the freezing point temperature for the solid carbon dioxide. Undersuch circumstances, the solid carbon dioxide will vaporize and apressure will soon build up within the container to a level sufficientto open the pressure relief valve 62. Carbon dioxide gas will bereleased into the vapor filled tanks through the outlet pipes 64 todisplace the vapors from the tanks through the relief opening 24 at thetop. Carbon dioxide vapors being heavier than the hydrocarbon vaporswill tend to stratefy at a level below the hydrocarbon vapors wherebythe vapors vented from the tank will be formed principally of thenatural gas although some carbon dioxide will be mixed in because ofturbulence and diffusion. Nevertheless, the major proportion of thenatural gas vapors will thus be displaced from the tank by the releasedcarbon dioxide to render the atmosphere within the tank inert.

When the tank is to be refilled with the liquefied natural gas, the tankcan be purged to remove the carbon dioxide but purging is not necessarysince an inert atmosphere is desirable. When filled with the liquefiednatural gas, or the like liquefied gas, the cold will stop furthervaporization of the carbon dioxide and, in fact, it will causesolidification of carbon dioxide vapors remaining within the containerwhereby the pressure in the container will drop rapidly to close thepressure relief valves 62 communicating the interior of the containerwith the tank. Thus the tank can be filled with liquefied gas and solidcarbon dioxide can be introduced into the container in an amount to makeup the solids lost by vaporization.

The pressure gauge 52 can be used to indicate the pressure conditionsexisting within the container :for purposes of control or for purposesof giving some indication as to conditions existing.

It will be apparent from the foregoing that I have provided a simple buteffective means for automatically controlling the atmospheric conditionsexisting within an enclosed space to effect removal of vapors underpredetermined conditions. The principal operation is quite simple andeffective in that it relies upon the use of an inerting material in theform of a liquid or solid which is capable of remaining in the liquefiedor solid from at the temperatures of the cargo liquid stored in the tankbut which is converted to a gaseous state when the tank rises to atemperature above the vaporization temperature of the inerting materialat the pressure for which the valve is set. Thus, the inerting materialremains for an indefinite period of time within the container and isavailable at all times to inert the tank when, for one reason oranother, the temperature rises within the tank, usually upon the removalof the liquid cargo.

These conditions can be met in the use of liquefied nitrogen as theinerting liquid when the liquid cargo in the tank is maintained at atemperature below the boiling point temperature for nitrogen at thepressure conditions existing. Similarly, other liquefied gases can beused as the liquid cargo in combination with solid carbon dioxide andthe like inerting or purging material as long as the describedconditions are met.

It will be understood that changes may be made in the details ofconstruction, materials and operation without departing from the spiritof the invention, especially as defined in the following claims.

I claim:

1. In the storage of a cold liquefied gas, an insulated tank having aninlet and an outlet for filling and discharging the liquefied gas and avapor outlet for bleeding vapors from the tank, a compartment of smalldimension in the bottom of the tank and sealed from the tank, pressureresponsive means commimicating the interior of the compartment with theinterior of the tank, and means for introducing a material into thecompartment having a critical gasification temperature above thetemperature of the liquefied gas in the tank whereby the material ismaintained in a condensed state within the compartment when liquefiedgas is in the tank and whereby it is converted to a gaseous state whenthe tank rises in temperature upon removal of the liquefied gas toactivate the pressure responsive means enabling the escape of thereleased gases into the tank to purge the tank of vapors therein.

2. In the storage of a cold liquefied gas, an insulated tank, means forfilling and discharging liquid from the tank and means in communicationwith the top of the tank for venting vapors from the tank, a housing ofsmaller dimension in the bottom of the tank and sealed from the interiorthereof, means responsive to pressure within the housing forcommunicating the interior of the housing with the interior of the tank,and means for introducing a material into the housing having a criticalgasification temperature above the temperature of the liquid whereby thematerial is maintained within the housing in a stabilized state whenliquid is in the tank and is converted to a gas when the tank rises intemperature upon the elimination of liquid therein to increase thepressure within the housing whereby the formed gas is introduced intothe tank for purging vapors therefrom.

3. A storage tank as claimed in claim 2 in which the means forintroducing the material into the sealed housing comprises an outlet ofsubstantial dimension in the housing and a tubular connection betweenthe outlet and the exterior of the tank and means for closing thetubular member for effecting a sealing relationship upon theintroduction of material.

4. A storage tank as claimed in claim 2 in which the housing is formedwith a plurality of openings and in which pressure responsive valvescommunicate the open ings with the interior of the tank, said valvesbeing normally closed and which open in response to predeterminedmaximum pressure to communicate the interior of the housing with thetank.

5. A storage tank as claimed in claim 2 in which the liquefied gascomprises liquefied natural gas and in which the material introducedinto the sealed housing is formed of an inert gas whereby the release ofthe gas from the housing into the tank operates to inert the tank.

6. A storage tank as claimed in claim 2 in which the liquid is aliquefied natural gas having a temperature below about 240 F. and inwhich the materialA introduced into the housing comprises solid carbondioxide.

7. In the storage of a cold liquid in an insulated tank of largecapacity having an inlet and outlet for lling and discharging theliquefied gas from the tank and a relief opening for the passage ofvapors from the tank, providing a housing in the bottom portion of thetank which is sealed from the tank but which has a pressure responsivemeans communicating the interior of the housing with the interior of thetank, and introducing a material into the sealed housing having acritical gasification temperature at a temperature above the liquefiedgas in the tank whereby the pressure within the housing remains belowthe predetermined pressure for opening the valve when liquefied gas ispresent in the tank but wherein the pressure within the housing rises tothe predetermined pressure for opening the valve when liquefied gas isno longer in the tank and the housing heats up to the criticaltemperature for the material whereby the latter is converted to agaseous state and introduced into the tank for purging vapors therefrom.

8. The method as claimed in claim 7 in which the cold liquid comprises aliquefied natural gas.

9. 'Ihe method as claimed in claim 7 in which the material introducedinto the housing for gasification comprises a material formed of aninert gas.

10. The method as claimed in claim 9 in which the material introducedinto the sealed housing comprises solid carbon dioxide.

References Cited in the file of this patent UNITED STATES PATENTS662,217 Brady Nov. 20, 1900 1,858,610 Banning May 17, 1932 1,885,769Schlumbohm Nov. 1, 1932 2,501,611 Nicholson Mar. 21, 1950 2,650,478Brown Sept. 1, 1953 2,668,419 Mapas Feb. 9, 1954

